The human intervertebral junction is characterized principally by an intervertebral disc interposed between adjacent vertebral surfaces. The size and configuration of discs vary between the six discs of the cervical region, the twelve discs of the thoracic region, six of the lumbar region and one disc between the sacrum and coccyx.
Intervertebral discs are neither homogeneous nor static. Changes to a disc can affect the vertebral column activity significantly. The intervertebral disc is a complex structure where its dynamic properties result from the interaction of a central, gelatinous nucleus pulposus encircled by a tough, fibrous, semielastic annulus fibrosus. Further, thin cartilage endplates and vertebral body ring apophyseal attachments of the annulus fibrosus join the disc to the vertebrae craniad and caudad to the disc. Although the nucleus pulposus is gelatinous and somewhat fluid while the annulus fibrosus comprises circularly arranged fibers, the border between these components is not distinct in a healthy adult disc. Any distinction is less apparent in a damaged disc where tissues are intermingled in a gradual transition layer.
The annulus fibrosus is composed of concentric layers of fibrocartilage, in which collagen fibers are arranged in parallel strands running obliquely between vertebral bodies. The inclination is reversed in alternate layers thereby crossing over each other obliquely. In children and adolescents, the nucleus pulposus is an amorphous colloidal mass of gelatinous material containing glycosaminoglycans, collagen fibrils, mineral salt, water and cellular elements. The nucleus pulposus has an important function in nutrition of the disc and contributes to the mechanical ability of the disc to act as a shock absorber and allow flexibility. The nucleus pulposus is normally under pressure and is contained within an ovoid cavity formed laterally by the annulus fibrosus and bounded by thin plates of hyaline cartilage endplates covering the adjacent vertebrae.
The intervertebral discs form about one-quarter the length of the vertebral column in a healthy adult human. Discs are thickest in the cervical and lumbar regions, where the movements of the vertebral column are greatest. The vertebral column, including the intervertebral discs, undergo various morphological and biochemical changes over time, such as dehydration of the discs and concaving vertebral bodies. As a result, the size and configuration of the disc components vary considerably from person to person.
Lower back injuries and chronic back pain are a major health problem resulting not only in a debilitating condition for the patient, but also in consuming a large proportion of funds allocated for health care, social assistance and disability programs. Disc abnormalities and pain may result from trauma, repetitive use in the workplace, metabolic disorders, inherited proclivity or aging. The existence of adjacent nerve structures and innervation of the disc are very important issues in respect to patient treatment for back pain.
Common disorders of the intervertebral disc include localized tears or fissures in the annulus fibrosus; disc herniations with contained or escaped extrusions of the nucleus pulposus; and chronic circumferential bulging of discs. For most patients, however, a well-defined abnormality cannot be found to solely explain the cause of the low back pain, making treatment and pain management very difficult. Since isolating a specific anatomic disorder as the sole cause of pain is rare, most patients are merely treated symptomatically to reduce pain, rather than receiving treatment to eliminate the cause of the condition.
One course of pain may be attributed to the structure of the annulus fibrosus. The annulus fibrosus is thinner nearer to the posterior than to the anterior margin of the disc, and many disc ruptures occur in the posterior region thereby exerting pressure on the adjacent nerve fibers causing pain. The pain experienced by the disc exerting pressure on the adjacent nerves is characterized by referred pain, or pain felt predominantly elsewhere in the body where the affected nerve travels. A common example of this is sciatica where an intervertebral disc exerts pressure on the sciatic nerve.
Another cause of pain resulting from disc pathology is chemically induced pain. The nucleus pulposus contains chemicals that may induce pain if contact is made with certain nerve structures. If an intervertebral disc is herniated severely enough that a portion of the nucleus pulposus is extruded from the disc, and the portion comes in contact with an adjacent nerve, chemically induced pain can be felt. This is also a cause of sciatica.
Increasingly, evidence suggests that the source of back pain in many patients is a result of nerves within the degenerated disc itself or nerves that have grown into the disc in concordance with disc injury. For example, as documented by Jonathan C. Houpt, BA, Edison S. Conner, MD, and Eric W. McFarland in “Experimental Study of Temperature Distributions and Thermal Transport During Radio frequency Current Therapy of the Intervertebral Disc”, Spine. 1996; 21(15), 1808-1813, afferent innervation of the outer half of the annulus fibrosus has been established whereas the nucleus pulposus contains no nerves or blood vessels. Pain response has been widely reported in response to specific stimulation of the outer layers of the annulus fibrosus. In another study documented by A. J. Freemont, “Nerve ingrowth into diseased intervertebral disc in chronic back pain”, The Lancet. 1997; 350, 178-181, nociceptive nerves were found ingrown deeper into the disc, as far as the nucleus pulposus, in association with disc degeneration. The pain experienced from nerves in a damaged intervertebral disc is more localized to the spine. The stimulation can be both mechanical and chemical. Some patients may feel a combination of back pain and referred pain indicating that pain is being transmitted both from nerves in the disc and from impinged nerves adjacent to the disc.
Where patients are diagnosed with clear chronic discogenic pain (i.e. pain originating from a disc), complete surgical removal of the intervertebral disc (called discectomy) and fusion of the adjacent vertebrae is often carried out with success rates over 80% in measurable pain reduction after surgery. Such major surgical procedures are highly invasive, expensive and involve significant risk. Furthermore motion is impeded once the vertebrae are fused and there may be adverse mechanical effects on the adjacent remaining discs.
To alleviate some of the disadvantages of open-surgery discectomy, percutaneous methods of removing the disc or part of the disc have been practiced. Methods that remove part of the nucleus pulposus are designed to decrease the volume in order to reduce internal disc pressure thus reducing external pressure exerted on adjacent nerves. Examples of such methods that include mechanical means can be found in, for example, U.S. Pat. No. 4,369,788 to Goald that describes the use of a mechanical device for use in microlumbar discectomy, and in U.S. Pat. No. 5,201,729 to Hertzmann et al. that describes a percutaneous method of discectomy using a laser. Other methods of removing the disc or part of the disc include chemically dissolving the nucleus pulposus using the enzyme Chymopapain. U.S. Pat. No. 6,264,650 to Hovda et al. describes a method of vaporizing a portion of the nucleus pulposus using radio frequency electrical current. These prior art methods have shown variable success and there are several advantages of percutaneous procedures over open surgical discectomy and vertebral fusion including less trauma to the patient, preserved spinal movement, less disruptive effect on adjacent discs, less risk of infection and less risk of accidental injury. However, these methods cause damage to the nucleus pulposus, which is essential to the maintenance of the disc. Further, the damaged annulus fibrosus is not treated.
Due to the pain reduction success of surgical discectomy, less drastic means of denervating rather than surgically removing the disc are of significant interest. To denervate is to intervene with the transmission of a sensory signal in a nerve. A denervated disc does not cause discogenic pain and the disc is left intact to preserve its mechanical function. Denervating the disc especially by using percutaneous probes is much less invasive, less costly and less risky. The procedure is also simpler to administer and does not require the fusing of adjacent vertebrae thereby better preserving the patient's freedom of movement.
To destroy nerve cells in the annulus fibrosus, the prior art includes probes that emit various forms of energy from within the nucleus pulposus such as, radio frequency electric current, microwave or thermal energy. It appears that the disc is devoid of temperature sensing neurological structures, probably since the disc is at core body temperature, and only mechanical and chemical stimulus-sensing nociceptors exist in the annulus fibrosus.
U.S. Pat. No. 5,433,739 to Sluijter et al. describes a method of relieving back pain through percutaneous insertion of a needle or electrode into the center of the intervertebral disc within the nucleus pulposus under fluoroscopy or other imaging control. The U.S. Pat. No. 5,433,739 patent describes the heating of the outer layers of the annulus fibrosus to a temperature that is lethal to the nerve structures thereby denervating the disc to relieve discogenic pain. The temperature of the tissue is increased by applying high frequency electric current through the tissue.
Radio frequency electrodes including an insulated shaft with an exposed tip conducting radio frequency current are commonly used in neurosurgery, anesthesiology and cardiology to lesion neural tissue. A second dispersive electrode with large surface area is placed elsewhere on the patient's body surface to complete the circuit. The intensity of radio frequency current at the exposed tip causes heating of the adjacent tissue. When the temperature increases sufficiently, the tissue is coagulated. The temperature that is sufficient to coagulate small unmyelinated nerve structures is 45° C., at which point direct interruption of the nerves occurs by the formation of a lesion. Thus, the transmissions of pain signals are blocked.
It is well known to those skilled in the art that percutaneous access to an intervertebral lumbar disc involves either a posterolateral approach or an anterior approach. The anterior approach is more invasive than the posterolateral approach because of the organs in the abdominal and pelvic cavities. The most common percutaneous approach to the lumbar disc, to those skilled in the art, is to insert a needle or tube posterolateral to the disc, just lateral of the zygapophyseal joint, inferior to the spinal nerve and into the posterolateral region of the annulus fibrosus.
In accordance with U.S. Pat. Nos. 5,980,504; 6,007,570; 6,073,051; 6,095,149; 6,099,514; 6,122,549; 6,126,682; 6,258,086 B1; 6,261,311 B1; 6,283,960 B1; and 6,290,715 B1 to Sharkey et al. to permit percutaneous access to the posterior half of the nucleus or to the posterior inner wall of the disc, a flexible heating element may be inserted into the nucleus pulposus through a hollow tube that has been pierced through the annulus fibrosus. The flexible heating element has sufficient rigidity to be advanced longitudinally under force through the nucleus pulposus while having flexibility to be compliant to the inner wall of the annulus fibrosus. The heating element is guided by sliding contact with the inner wall and ideally should not puncture or damage the annulus fibrosus during positioning. Another embodiment of the U.S. Pat. No. 6,258,086 B1 patent is a flexible probe that contains an activation element on the distal portion that changes the shape of the probe once it is in the nucleus pulposus. According to the Sharkey et al. patents, the flexible heating elements operate to denervate the outer layers of the annulus fibrosus as well as modulate the collagen in the annulus fibrosus by applying heat. Raising the temperature above about 60° C. will break structural bonds of collagen fibers causing them to contract and tighten. This collagen tightening effect is lost once the temperature of the collagen is raised above about 75° C. where the fibers loosen, resulting in zero net volume change.
It is also known to insert an energy delivery device in to the nucleus pulposus, or the transition zone between the nucleus pulposus and the inner wall of the annulus fibrosus, in order to transfer heat from the nucleus pulposus to selected areas of the annulus fibrosus. These known methods do not allow applying energy to these target areas without unnecessarily entering and applying energy to the nucleus pulposus.
There are several disadvantages to unnecessarily entering into and applying energy to the nucleus pulposus. Disadvantages include disrupting the metabolism of the intervertebral disc, impeding the healing processes, and altering the structure of healthy tissues of the disc such as the cartilage endplates, nucleus pulposus and areas of the annulus fibrosus that are not targeted. Additional disadvantages include causing unnecessary physical damage, increasing the risk of discitis, and potentially removing nuclear material that can come in contact with other adjacent tissues thus causing biochemical damage. If energy is applied to the target areas of the intervertebral disc without inserting a device into the nucleus pulposus, these disadvantages are avoided.
It has also been found that the nucleus pulposus has very high heat conductivity probably due to its gelatinous and high water content characteristics. The manipulation of prior art probes within the nucleus pulposus to position an energy emitter close to the site of the annulus injury appears to be largely unnecessary since heat is readily conducted throughout the nucleus pulposus no matter where the probe is located within the nucleus. Clearly the prior art is less than optimal since these techniques involve damage to the annulus fibrosus which is pierced to gain access to the nucleus pulposus. Even once disposed within the nucleus pulposus, the delivery of sufficient energy to denervate the desired area is inhibited by dissipation caused by the high heat conductivity of the nucleus pulposus. Using the methods described in the prior art the annulus is punctured thus spreading heat to all parts of the nucleus pulposus.
Another prior art device, described in PCT publication number WO 01/45579 to Finch et al., avoids entry into the nucleus pulposus by extending a probe a relatively long distance within the annulus fibrosus conforming to an azimuthal course defined by the natural lamina of the annulus fibrosus. Therefore the elongate probe extends between layers of the annulus fibrosus to deliver heat energy to a large segment of the annulus fibrosus without entering the nucleus pulposus or directly heating the nucleus itself. However, tunneling through the layers of the annulus fibrosus can significantly damage the annulus fibrosus and layers may peel apart. Surgical manipulation involves inaccuracies and in the case of ruptured or fissured discs, tunneling may cause further deterioration of the annulus fibrosus.
There is interest among researchers that the application of high frequency current without a rise in temperature alters nerve function to relieve pain and also may cause collagen production to increase and stimulate healing of the annulus fibrosus. The use of high frequency current without heating to relieve pain by modifying neural tissue is described in U.S. Pat. Nos. 5,983,141; 6,161,048; 6,246,912; and 6,259,952 to Sluijter et al. These patents describe the use of a modified signal wave that includes rest periods to allow heat to dissipate. The modified high frequency signal is applied to the patient using a single active electrode and a ground electrode attached to the skin of the patient. These prior art patents ('141, '048, '912, and '952) do not discuss using high frequency current to increase collagen production nor do they discuss this application in the intervertebral disc. The prior art inventions that are specifically designed for treatment of intervertebral discs (Sharkey et al. U.S. Pat. Nos. 5,980,504; 6,007,570; 6,073,051; 6,095,149; 6,099,514; 6,122,549; 6,126,682; 6,258,086 B1; 6,261,311 B1; 6,283,960 B1; and 6,290,715 B1; Sluijter et al. U.S. Pat. No. 5,433,739; Finch PCT publication number WO 01/45579) do not discuss the application of high frequency current without a rise in temperature to alter nerve function to relieve pain or to cause collagen production to increase. The advantages of non-thermal application of high frequency electrical current to treat intervertebral discs include reduced risk of thermal damage, increased production of collagen to strengthen the annulus fibrosus, and reduced discogenic pain while stimulating the healing processes.
It is an object of the present invention to provide a device that can treat intervertebral disc disorders by applying energy to the injured or degenerated areas of the annulus fibrosus, without the need to tunnel through or otherwise severely damage the annulus fibrosus and without the need to enter into or heat the nucleus pulposus.
It is a further object of the present invention to deliver energy to a relatively large elongate segment of the annulus fibrosus without the need to physically insert a probe into the entire segment thereby avoiding the risk of further deteriorating the treated segment of the annulus fibrosus.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.